Intervalve coupling and like circuit



June 13, 1939. J gw c ET AL 2,161,959

INTERVALVE COUPLING AND LIKE CIRCUIT Filed July 8, 1936 INVENTORS JOHNHARDWICK ERIC L. C. WHiTE ATTORNEY Patented June 13, 1939 UNITED STATESPATENT OFFICE INTERVALVE COUPLING AND LIKE CIRCUIT Application July 8,1936, Serial No. 89,668 In Great Britain July 9, 1935 6 Claims.

The present invention relates to circuits suitable for use as intervalvecouplings and for like purposes. I

In thermionic valve amplifiers which are required to provide uniformamplification of electrical variations, such as television picturesignals, including components of frequencies within a wide range, it isoften found that stray capacities such as the inter-electrode capacitiesof the amplifier Valves cause the response of the amplifier to departfrom uniformity at the higher frequencies of the range.

Now in many amplifiers, such as those employed for television purposes,for example, it is often desired that no relative phase displacement ofoscillations at different frequencies shall take place; in some cases,however, phase displacement the magnitude of which is proportional tofrequency is permissible, since such a displacement is merely an advanceor delay of the applied signal as a whole with respect to time, which isof no consequence. In such amplifiers, it is inconvenient to compensatefor a falling-off of response at high frequencies by the use ofattenuation-correcting means, since such means introduce relative phaseshift which is not proportional to frequency, and the correction of suchphase shift as a matter of considerable practical difiiculty.

It is the object of the present invention to provide a circuit forintervalve coupling and like purposes, by the use of which thedifficulties discussed above can be avoided or reduced.

The present invention accordingly provides a four-terminal networksuitable for use, for example, as an intervalve coupling, said networkhaving an input shunt capacity and an output shunt capacity which isgreater than twice or less than one-half the input shunt capacity, inwhich two inductance coils are arranged in series between said input andoutput capacities, and in which a resistance is connected eifectively inshunt in said network at the junction of said coils, the inductances ofsaid coils and the magnitude of said resistance being so chosen eitherthat the attenuation introduced by said network is approximately uniformover a substantial range of frequencies, or that, within said range, thephase shift introduced is substantially proportional to frequency.Either or both of the said capacities may be constituted wholly or inpart by distributed capacity such as an interelectrode capacity of athermionic valve, a photoelectric cell, or the like.

The invention further provides a circuit ar- 55 rangement comprisingfirst and second thermionic valves, having their cathodes connectedtogether and to the negative terminal of an anode current source, inwhich two inductance coils are connected in series between the anode ofsaid first valve and the control grid of the second, and in which aresistance is connected between the Junetion of said coils and thepositive terminal of said source, the effective anode-cathode andcontrol grid-cathode capacities of said valves respectively forming withsaid coils and said resistance a fourterminal network which providesapproximately uniform attenuation over a substantial range of frequency,or in which over a substantial range of frequency, the phase shiftintroduced is substantially proportional to frequency. In a modificationthe inductances of said coils and the magnitude of said resistance liebetween those required to provide substantially uniform attenuation andthose which satisfy the requirement that the phase shift introducedshall be proportional to frequency.

In a modification of the invention as set forth in the precedingparagraph, said first valve is replaced by a photoelectric cell.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawing.

Referring to the drawing, a screened-grid valve I has its cathodeconnected to earth through a biasing resistance 2 shunted by a by-passcondenser 3, its control grid connected to one of two input terminals 4,and its screen grid connected to a point at a suitable positivepotential in a source of space current (not shown), the negativeterminal of which is earthed.

The anode of valve I is connected through inductance coils L1 and L2,and a coupling condenser 5, all in series, to the control grid of atriode valve 6. The cathode of this valve is earthed through biasresistance 1 and by-pass condenser 8 in shunt, and its anode isconnected to a point in the space current source at a suitable positivepotential through resistance 9. A grid leak i0 is connected between thegrid of valve 6 and earth.

The junction point of coils L1 and L2 is connected to a point at asuitable positive potential in the space current source throughresistance II, which serves as the anode resistance of valve I.

Now the anode-to-screening grid capacity of valve I may be representedby a capacity C1 between anode and earth, since the screening grid is atearthy potential (that is at a potential which is substantially constantrelatively to earth).

2. 1 The control-grid-cathode capacity of valve 6 may be'represented bya capacity C2 connected between the join of coil L2 and condenser 5, andearth. If

it now be assumed that a potential difference is applied to terminals 4which is of uniform amplitude at all frequencies, the potentialdifference E set up across capacity C: at low frequencies isapproximately equal to 'ioR, where in is the output current of valve I,and R is the magnitude 0 resistance I I. At a frequency w, v g

i R V E- A i where A is in general a function of w and is theattenuation of the network constituted by C1, C2; L1, L2 and resistanceI I. v

By straightforward analysis, which it is not necessary to set out here,there can be derived 7 this expression containing even powers of w upexpression for to a A similar expression can be obtained for (E do:

being the phase angleof E with respect to is.

The magnitude R of resistance ll determines the gain of the valve. l, C1is determined mainly by the construction of valve I, and the values ofL1, 11 and b are then chosen in'a manner which will now be outlined,either so asto make A as "uniform as possible over a desired frequencyrange, or to make as constant as possible with frequency over thedesired range, or as it is not possible to satisfy completely both theserequirements, the invention' enables 'a satisfactory compromise to beachieved. a If in the expressionforAmentioned above, the coefiicient ofw is equated to zero, the followingexpression is obtained: V

' c;n 1+afi r 2(1+ab) and equating to zero the coefficient of (o in thed dw gives 7 1- 2'a a a 7 "mi; In order to make the phase'shiftintroduced substantially proportional to frequency, a is given j a valuesuch that the influence of terms in m approximately satisfied, where kisa constant. The value for b follows from (ii), and the value for L1 isobtained by substitution in (i) the designof the circuit-is thencompletely fixed.

' 'In the case of a television amplifier, a convenient value for a is2.5, that is, 02:2.5 C1 and C1 and Czare so chosen (for example, bysuitable selection of valves) that this condition is substantiallysatisfied; when (1 2.5, it is found that not only is the phase shiftintroduced substantially proportion to frequency, within a predeterminedrange of frequency, but that, within that range, the attenuation isapproximately uniform. The required value for a can be obtainedbyaugmenting C1 or C2 artificially, but if the circuit is required toWork up to a very high frequency, such an addition to shunt capacityshould preferably be avoided, since the smaller C1 and C2 can be made,the higher the frequency up to which the-attenuation will be uniform.

'In an example, C1 and C2 have the values 16 and 40 micro-microfaradsrespectively, and R is 2000 ohms; in this case, L1 and L2 are given thevalues 45 and 150 micro-henrys respectively.

From an examination of (ii) above, it will be clear that b is positiveonly when a is greater than 2 or less than one-half, and since ab 1 itis clear that both coils L1 and L2 only have finite. positive valueswhen a 2. In the arrangement described above, a 2; when,however,' a,'-the coils L1 and L2 are interchanged in position. I

In order to convert the arrangement illustrated in the accompanyingdrawing into a D. C. amplifier, the condenser 5, maybe'replaced by afloating battery having its negative terminal connected to the controlgrid of valve 6, the leak Ill being then omitted. The resistance IIshould preferably be one having a low self-capacity; if desired,however, there may be connected in series between resistance H and thejoin of coils L1 and L2 either a coil of. an inductance equal to 0.4CSRor an inductance of magnitude Celt shunted by a resistance of magnitudeR, Cs in bothjcases being the self-capacity of resistance I I; botharrangements provide a measure of compensation for the self-capacity ofresistance II,

the compensation provided by thelatter method being more complete. Ifdesired, damping resistances may be connected in shunt with coils L1 andL2 respectively to prevent amplitude overswing due to transients; theseresistances may have values from 5 to 20 times R, and account must betaken of their presence in designing the circuit by increasing thevalues of L1 and L2.

The network C1, C2, L1, L2, R is of the low.- pass filter type. Theequivalent band-pass filter can be constructed by connecting condensersin series with coils L1 and L2, and shunting the capacities C1 and C2 byinductances; the magnitudes of these additional elements can bedetermined by an extension of the procedure outlined in the descriptionof the arrangement shown in the accompanying drawing. In the case inwhich'two valves are coupled by a band-pass filter in accordance withthis invention, the two valves serving as carrier-frequency amplifiers,the introduction of a certain amount of phase shift may similarprocedure maybe followed in'the design of low-pass networks according tothe invention for use, for example, as intervalve coupling cir-.'

cuits for audio-amplifiers, and for other purposes where phaseshifts arerelatively unimportant,

but where uniform attenuation is a'desideratum.

Although the invention has been particularly described in itsapplication to intervalve coupling circuits, it can clearly be appliedfor many other purposes; for example, a network according to theinvention can be employed for coupling a photo-electric cell to athermionic valve. Many further modifications and applications of theinvention, within the scope of the appended claims, will occur to thoseversed in the art.

We claim:

1. An electrical network comprising in combi-' nation input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being greater than twice said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities, a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that the attenuation introduced by said network isapproximately uniform over a substantial range of frequencies.

2. An electrical network comprising in combination input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being less than one half said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities,a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that the attenuation introduced by said network isapproximately uniform over a substantial range of frequencies.

3. An electrical network comprising in combination input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being greater than twice said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities, a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that over a substantial range of frequencies, the phase shiftintroduced is substantially proportional to frequency.

4. An electrical network comprising in combination input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being less than one half said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities, a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that over a substantial range of frequencies, the phase shiftintroduced is substantially proportional to frequency.

5. An electrical network comprising in combination input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being greater than twice said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities, a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that a compromise is efiecied between causing the attenuationintroduced by said network to be approximately uniform over asubstantial range of frequencies and causing the phase shift introducedto be substantially proportional to frequency over said range offrequencies.

6. An electrical network comprising in combination input and outputmeans, said means having shunt capacity, said shunt capacity of saidoutput means being less than one half said shunt capacity of said inputmeans, inductance coils arranged in series between said input and saidoutput shunt capacities, a resistance connected effectively in shunt insaid network at a junction between said inductance coils, the inductancevalues of said inductance coils and the value of said resistance beingchosen so that a compromise is effected between causing the attenuationintroduced by said network to be approximately uniform over asubstantial range of frequencies and causing the phase shift introducedto be substantially' proportional to frequency over said range offrequencies.

JOHN HARDWICK. ERIC LAWRENCE CASLING WHITE.

